1. Field of the Invention
The present invention relates to an EL (electroluminescence) display device formed of a semiconductor element (an element using a semiconductor thin film) built into a substrate, and to an electronic device (an electronic device) having the EL display device as a display.
2. Description of the Related Art
Techniques of forming a TFT on a substrate have been widely progressing in recent years, and development of applications thereof to an active matrix type display device are advancing. In particular, a TFT using a polysilicon film has a higher electric field effect mobility (μFE) than a TFT using a conventional amorous silicon film, and high speed operation is therefore possible. As a result, it becomes possible to perform pixel control, conventionally performed by a driving circuit external to the substrate, by the driving circuit formed on the same substrate as a pixel.
This type of active matrix display device has been in the spotlight because of the many advantages which can be obtained by incorporating various circuits and elements on the same substrate in this type of active matrix display device, such as reduced manufacturing cost, display device miniaturization, increased yield, and higher throughput.
In a monolithic type display device having a pixel portion and a driving circuit for driving the pixel portion on the same substrate, the driving circuit is formed in the periphery of the pixel portion, and therefore, compared to forming only the pixel portion on the substrate, the required substrate size becomes larger by the size of the driving circuit. Consequently, the number of display devices which can be cut out of one substrate changes with how small the exclusive surface area of the driving circuit can be made.
In particular, in a display device whose pixel portion has a diagonal of 1 inch or less, it is necessary to load the driving circuit on an extremely small substrate, and the exclusive surface area of the driving circuit imparts a large influence on the substrate size. However, regardless of the size of the pixel portion, the driving circuit functions are the same, and in order to form a circuit with identical functions in a very small area, various factors such as increasing the TFT characteristics and techniques of miniaturization become key points.